1. Field of the Invention
The present invention relates to a vacuum booster used for the servo operation of a brake master cylinder for a vehicle. In particular, the present invention pertains to an improved vacuum booster, wherein a booster piston is located in a booster shell to divide the interior of the booster shell into a front vacuum chamber communicating with a vacuum source and a rear operating chamber, wherein a valve piston fitted into a valve cylinder so as to be slidable forward and backward, an input rod that connects the front end thereof to the valve piston, a control valve that, in accordance with the forward and backward movement of the input rod between the valve piston and the valve cylinder, alters the communication of the operating chamber with either the vacuum chamber or the atmosphere, and an input return spring that impels the input rod backward are arranged in the valve cylinder that communicates with the booster piston, wherein the control valve is constituted by a ring-shaped vacuum introduction valve seat formed in the valve cylinder, an atmosphere introduction valve seat formed in the valve piston and positioned inside the vacuum introduction valve seat, a valve element that includes: a ring-shaped attachment bead portion securely supported at the inner wall of the valve cylinder; an elastic cylinder axially extending from the attachment bead portion; and a valve portion connected, flange-like, to the front end of the elastic cylinder and located facing the vacuum introduction valve seat and the atmosphere introduction valve seat, on which the valve portion is to be seated, and wherein a first port that communicates with the vacuum chamber is opened to the outer wall side of the vacuum introduction valve seat, and a second port that communicates with the operating chamber is opened between the vacuum introduction valve seat and the atmosphere introduction valve seat, thus enabling the inside of the valve portion to communicate with the atmosphere.
2. Description of the Related Art
Vacuum boosters are well known, as is disclosed, for example, in JP-B-58-48923U.
The periphery of the control valve of the vacuum booster disclosed in the referenced publication is shown in FIG. 6. As is apparent from FIG. 6, a control valve 38 of a conventional vacuum booster includes: a ring-shaped vacuum introduction valve seat 30, which is formed in a valve cylinder 10; an atmosphere introduction valve seat 31, which is formed on a valve piston 18 that is connected to an input rod 20 and is located inside the vacuum introduction valve seat 30; a valve element 34, which includes a ring-shaped attachment bead portion 34b that is securely supported by the inner wall of the valve cylinder 10, an elastic cylinder 34c that extends forward from the attachment bead portion 34b, and a ring-shaped valve disk 34a that is connected, flange-like, to the front end of the cylinder 34c and is positioned opposite the vacuum introduction valve seat 30 and the atmosphere introduction valve seat 31 on which it is seated; and a valve spring 36, which impels the valve disk 34a in the direction in which it is seated on the vacuum introduction valve seat 30 and the atmosphere introduction valve seat 31. A first port 28, which communicates with the vacuum chamber 2, opens to the outer wall-side of the vacuum introduction valve 30, and a second port 29, which communicates with the operating chamber 3, is opened between the vacuum introduction valve seat 30 and the atmosphere introduction valve seat 31. An atmosphere introduction port 39, which communicates with the inner wall of the valve disk 34a, is formed at the rear of the valve cylinder 10. In addition, an input return spring 41, which impels the input rod 20 to the rear, is retracted between the valve cylinder 10 and the input rod 20, while a booster piston 4 is integrally coupled with the valve cylinder 10.
Generally, for this vacuum booster, since the initial operating force exerted by the input rod 20 is determined in accordance with the set load of the input return spring 41, it is preferable that the set load of the input return spring 41 be as small as possible in order to reduce the required operating force.
Assume that the atmosphere introduction valve seat 31 is released from the valve portion 34a of the valve element 34 by moving the input rod 20 forward against the set load of the input return spring 41, that the valve portion 34a is seated on the vacuum introduction valve seat 30, and that a forward propulsive force produced by a pressure difference between the vacuum chamber 2 and the operating chamber 3 is exerted on the booster piston 4. Since in this state, atmospheric pressure acts on the front face of the valve portion 34a and a vacuum pressure, produced by the vacuum chamber 2, acts on the rear face, it is necessary for the set load of the valve spring to be satisfactorily large in order to prevent the propulsive force produced by the pressure difference from releasing the valve portion 34a from the vacuum introduction valve seat 30. However, when the set load of the valve spring 36 is large, accordingly, the set load of the input return spring 41 must also be large. Otherwise, when the operating force applied to the input rod 20 is released, the set load of the valve spring 36 will prevent the valve portion 34a from separating from the vacuum introduction valve seat 30. This factor prevents any reduction in the set load of the input return spring 41 of the conventional vacuum booster.
To resolve this problem, it is one objective of the present invention to provide a vacuum booster wherein a smaller set load is provided for a valve spring, and accordingly, the set load of an input return spring can be reduced, as can the initial operating force applied to the input rod 20.
To achieve the above objective, according to a first aspect of the invention, there is provided a vacuum booster, comprising:
a booster shell;
a booster piston accommodated in the booster shell to divide the interior of the booster shell into a front vacuum chamber communicating with a vacuum source and a rear operating chamber;
a valve cylinder connected to the booster piston;
a valve piston fitted into the valve cylinder so as to be slidable forward and backward;
an input rod connected to the valve piston at a front end thereof;
an input return spring urging the input rod backward; and
a control valve arranged within the valve cylinder, for changing over the communication of the operating chamber with one of the vacuum chamber and the atmosphere in accordance with the forward and backward movement of the input rod, the control valve including:
a ring-shaped vacuum introduction valve seat formed in the valve cylinder;
an atmosphere introduction valve seat formed in the valve piston and disposed inside the vacuum introduction valve seat;
a valve element having: a ring-shaped attachment bead portion securely held with respect to an inner wall of the valve cylinder; an elastic cylinder axially extending from the attachment bead portion; and, an annular valve portion connected in a flange shape to a front end of the elastic cylinder and facing the vacuum introduction valve seat and the atmosphere introduction valve seat, on which the valve portion is to be seated; and
a valve spring urging the valve portion in a direction that the valve portion is seated on the vacuum introduction valve seat and the atmosphere introduction valve seat, wherein a first port that communicates with the vacuum chamber is opened to an outer periphery side of the vacuum introduction valve seat, a second port that communicates with the operating chamber is opened between the vacuum introduction valve seat and the atmosphere introduction valve seat, and an inner periphery side of the valve portion is communicated with the atmosphere,
further wherein the valve portion is fitted with the valve cylinder so as to be slidably contacted with the inner wall of the valve cylinder, to define a front annular chamber and a read ring chamber within the valve cylinder, the front rind chamber being closed by a front face of the valve portion when the valve portion is seated on the vacuum introduction valve seat, the rear annular chamber being formed so that the rear of the valve portion is faced thereto, and
further wherein the front annular chamber communicates with the first port, and the rear annular chamber communicates with the second port.
According to the first aspect, when the input rod is moved forward against the set load of the input return spring, and when the atmosphere introduction valve seat is separated from the valve portion of the valve element while the valve disk is seated on the vacuum introduction valve seat, the vacuum pressure, which is transmitted from the first port to the front annular chamber of the valve cylinder, acts on the front face of the valve portion facing the front annular chamber, while the atmospheric pressure, which is transmitted from the second port to the rear annular chamber, acts on the rear face of the valve seat facing the rear annular chamber. Thus, not only in accordance with the set load of the valve spring, but also in accordance with the pressure difference between the front and rear annular chambers, the valve portion is urged in the direction in which it will be seated on the vacuum introduction valve seat. Therefore, the set load of the valve spring can be reduced by a value equivalent to the urging force produced by the pressure difference. And accordingly, only a small set load is required for the input return spring that impels the input rod to the rear, so that the load imposed by the initial operation of the input rod can be reduced.
According to a second aspect, in addition to the first aspect, the valve portion includes a sealing lip in the outer periphery thereof, bent rearwardly so as to be slidably contacted with the inner wall of the valve cylinder.
According to the second aspect, when the atmosphere introduction valve seat is separated from the valve portion of the valve element, and when the valve portion is seated on the vacuum introduction valve seat, the force with which the sealing lip is held closely against the inner wall of the valve cylinder is increased due to the pressure difference generated between the front and rear annular chambers, and between the front and rear annular chambers an airtight condition can be obtained.
In addition to the first and second aspect, according to a third aspect, a through hole communicating with the rear annular chamber and the first port is formed in the valve cylinder parallel to an axial line of the valve cylinder.
According to the third aspect, at the same time as the valve cylinder is formed, the through hole can also be formed by a core cylindrical pin. Thus, a valve cylinder having a communication path can be provided at a low cost.